Method of preparing electrophorettable polymer emulsions

ABSTRACT

Disclosed is a method of making a non-aqueous emulsion from which a polymer can be electrodeposited. A mixture is prepared of about 50 to about 150 parts by weight of a non-aqueous organic, non-electrolizable, non-solvent for the polymer with about 0.8 to about 1.2 parts by weight of a nitrogen-containing base which can be a tertiary amine, an imidazole, or mixture of a tertiary amine and an imidazole. To the mixture is added a solution of 1 part by weight of the polymer which can be a polyamic acid, a polyamide imide, a polyimide, a polyparabanic acid, a polysulfone, or a mixture of these polymers. The polymer is in a non-aqueous, organic, non-electrolizable aprotic solvent such as N-methyl-2-pyrrolidone.

BACKGROUND OF THE INVENTION

When a direct current potential is applied across a liquid mediumcontaining charged particles, the particles migrate towards theelectrode bearing the opposite charge and deposit thereon. This processis called electrophoretic deposition and is utilized to apply coatingsto conductive surfaces for electrical insulation, paint priming, andother purposes.

Electrophoretic deposition from non-aqueous media is accomplished usingpolymeric emulsions. The composition and formation of these emulsionscritically affect the emulsion stability, the particle size of theemulsion droplets, and ultimately, the quality and quantity of thedeposited coating. Until now, these polymeric emulsions have beenprepared by reacting an organic amine with a polymer in a solution toform an amine salt. The solution was then dispersed in a non-solventwhich formed the emulsion. The formation of the salt was believed to benecessary because it was thought that the polymer would not migrate tothe electrode unless a charge was placed on it. It was also thought thatthe reaction of the amine with the polymer to form the salt requiredheating the amine with the polymer, and that this could not be doneefficiently in the presence of the non-solvent. While good emulsionswere obtained, results were not totally satisfactory because largepolymeric particles precipitated and the size of the particles remainingin the emulsion were generally larger than was desirable for long-termstability and coating uniformity.

SUMMARY OF THE INVENTION

We have found that it is not necessary to react the amine with thepolymer prior to adding the polymer to the non-solvent. In fact,surprisingly, we have found that adding a solution of the polymer to anon-solvent containing the amine produces an emulsion that is finer inparticle size and has an improved long-term stability and depositionyield. That is, the emulsion of this invention has less tendency tobreak on storage, the coulombic yield and the physical yield per unittime are higher, and a smoother coating is produced because the particlesize is smaller

PRIOR ART

U.S. Pat. Nos. 4,053,444; 4,003,812 and 3,943,065 disclose reacting apolymer with a nitrogen-containing base to form a salt which is thenadded to a non-solvent to form an electrophorettable emulsion.

U.S. Pat. No. 4,019,877 discloses mixing a surfactant with a polymersolution and adding that mixture to a non-solvent to form anelectrophorettable emulsion of polyimides.

DESCRIPTION OF THE INVENTION

The polymers which can be electrodeposited using the process of thisinvention include polyamic acids, polyamide-imides, polyimides,polyparabanic acids, polysulfones, and mixtures thereof. Polyimides arethe preferred polymer because they work very well and are commerciallyimportant. Detailed structure of various suitable polyimides can befound in U.S. Pat. No. 4,053,444, herein incorporated by reference.

A solution is prepared of the polymer with a non-aqueous, organic,non-electrolizable, aprotic solvent. Suitable solvents include thenormally liquid organic solvents of the N,N-dialkylcarboxylamide class,preferably the lower molecular weight members of this class, such asdimethyl acetamide, dimethyl formamide, and N-methyl-2-pyrrolidone.Other suitable solvents include dimethyl sulfoxide and pyridine. Thesolvents can be used individually or in combinations of two or more. Thesolvents should be easily removable by heating in a drying tower oroven. The preferred solvent is N-methyl-2-pyrrolidone because it can beused with almost all of the above polymers. The solid content of thepolymer solution should be about 0.5 to about 5.0 weight percent basedon the total weight of the polymer in the solvent.

In the practice of this invention a mixture is prepared of about 50 to150 parts by weight of a non-aqueous organic, non-electrolizable,non-solvent for the polymer with about 0.8 to about 1.2 parts by weightof a nitrogen-containing base. The non-solvent for the polymer must notgas to any great extent at the electrodes due to electrolysis when avoltage is applied between them. Preferred non-solvents includenitroalkanes, acetonitrile, liquid aliphatic (straight and branchedchain) and aromatic ketones, such as, for example, acetone, methylisobutyl ketone, methyl ethyl ketone, methyl n-propyl ketone, diethylketone, mesityl oxide, cyclohexanone, methyl n-butyl ketone, ethyln-butyl ketone, methyl n-amyl ketone, acetophenone, methyln-hexylketone, isophorone, and diisobutylketone. The preferred ketone isacetone because it can be used with almost all polymer systems. Suitablenitrogen-containing bases include organic aliphatic and aromatic amines.Tertiary amines are preferred because they then provide a more stableemulsion and a higher electrodeposit yield. Examples of suitablenitrogen-containing bases include trimethylamine, triethylamine,N,N-dimethylbenzylamine, tri-n-propylamine, tri-n-butylamine,N-ethylpiperidine, N-allylpiperidine, N-ethylmorpholine,N,N-diethyl-m-toluidine, N,N-diethyl-p-toluidine, N-allylmoropholine,N,N-diethylaniline, and pyridine. Imidazoles such as, for example,imidiazole itself, 1-methylimidazole, 4-methylimidazole,5-methylimidazole, 1-propylimidazole, 1,2-dimethylimidazole,1-ethyl-2-methylimidazole, and 1-phenylimidazole can also be used,either alone or mixed with a tertiary amine. The preferrednitrogen-containing base is triethylamine because it provides stableemulsions with a high electrodeposit yield.

The electrodeposition composition is formed by adding the polymersolution to the solution of the non-solvent and the nitrogen base. Theelectrodeposition composition should contain about 1 part (by weight) ofthe polymer, about 29 to about 37 parts of solvent, about 0.8 to about1.2 parts of nitrogen-containing base, and about 50 to about 150 partsof the non-solvent. If less than 29 parts of solvent are used, theviscosity will be too high and precipitation of the polymer may occur,and if more than 37 parts of solvent are used, the polymer may stay insolution and not coat on the electrode. If less than 50 parts of thenon-solvent are used, electrocoating may also be inhibited because thepolymer may stay in solution, and if more than 150 parts of non-solventare used, the polymer may precipitate. The electrodeposition compositioncan be used in the same manner as prior electrodeposition compositions.(See U.S. Pat. Nos. 4,053,444; 3,943,065; and 4,019,877, hereinincorporated by reference). Small amounts of filler particles or finelydivided pigments or lubricants can be added to the composition ifdesired.

The following examples further illustrate this invention:

EXAMPLE 1

Two emulsions were prepared using a polyamicacid material as a 15 wt.%solution in N-methyl pyrrolidone (NMP) which was sold by the DuPont Co.under the trade designation "Pyre ML-RC5057." In emulsion A the amineand the Pyre ML were heated for 20 minutes at 40° C. prior to adding itto acetone. In emulsion B the polymer solution was added to the acetonewhich contained the amine. The composition of the emulsions was 10 gPyre ML, 62 g NMP, 0.4 g triethylamine (Et₃ N) and 178 ml of acetone.

Emulsion A had a white opaque color and some fibrous polymericprecipitate on the bottom of the beaker. Emulsion B was semi-transparentand slightly turbid with no precipitated polymer. These emulsions wereelectrolyzed at 50 Vdc for 60 seconds using aluminum electrodes 11/2"wide and immersed 2" with 1" separation. The yield of deposit foremulsion A was 31 mg and the yield on the anode of emulsion B was 36 mg.The coulombic yield for emulsion A was 63 mg/coulomb and the coulombicyield for emulsion B was 72 mg/coulomb. Samples of the emulsions werecovered and left to stand at room temperature for 2 months. While noprecipitate formed in emulsion B the precipitate which formed inemulsion A was about 10% of the total weight of the polymer.

EXAMPLE 2

Two emulsions were prepared using a 20 wt.% solution in dimethylformamide (DMF) of a polyparabanic acid polymer sold by Exxon Chemicalsunder the trade designation "PPA-M." These emulsions were prepared inthe same manner as emulsions A and B in Example 1. The emulsionscontained 5 g PPA-M, 39 g dimethyl formamide, 0.36 g triethylamine, and120 ml of acetone. Emulsion A was an opaque-white color with a polymerprecipitate that constituted about 20% of the total polymer weight.Emulsion B had a deep white color with no large precipitated particles.The emulsions were electrolyzed at 100 Vdc using aluminum electrodes asin Example 1. The yield of deposit for emulsion A was 20 mg and theyield for emulsion B was 36 mg. The coulombic yield for emulsion A was 7mg/coulomb and the coulombic yield for emulsion B was 37 mg/coulomb.

EXAMPLE 3

Two emulsions were prepared using a powder of polyphenyl sulfone sold byUnion Carbide Corporation under the trade designation "Radel-5000." Theemulsions were prepared in the same manner as emulsions A and B inExample 1. The composition of these emulsions was 1.38 g Radel-5000,58.6 g NMP, 0.69 g Et₃ N, and 148 ml of acetone. Both emulsions wereopaque and white in color. The emulsions were electrolyzed as inExample 1. The yield of emulsion A was 4 mg and the yield of emulsion Bwas 21 mg.

What is claimed is:
 1. A method of making a non-aqueous emulsion fromwhich a polymer can be electrodeposited comprising:(A) preparing amixture about 50 to about 150 parts by weight of a non-aqueous organicnon-electrolizable non-solvent for said polymer with about 0.8 to about1.2 parts by weight of a nitrogen-containing base, selected from thegroup consisting of tertiary amines, imidazoles, and mixtures thereof,(B) adding to said mixture a solution of about 0.5 to about 5% solidscontaining 1 part by weight of said polymer, said polymer being selectedfrom the group consisting of polyamic acids, polyamide-imides,polyimides, polyparabanic acids, polysulfones, and mixtures thereof, ina non-aqueous, organic, non-electrolizable, aprotic solvent.
 2. A methodaccording to claim 1 wherein said non-solvent is a ketone.
 3. A methodaccording to claim 2 wherein said ketone is acetone.
 4. A methodaccording to claim 1 wherein said non-solvent is acetonitrile.
 5. Amethod according to claim 1 wherein said solvent isN-methyl-2-pyrrolidone.
 6. A method according to claim 1 wherein saidnitrogen-containing base is triethylamine.
 7. A method according toclaim 1 wherein said emulsion is about 29 to about 37% solvent.
 8. Amethod according to claim 1 wherein said polymer is a polyimide.
 9. Amethod according to claim 1 wherein said polymer is a polyamic acid. 10.A method according to claim 1 wherein said polymer is a polyamide-imide.11. A method according to claim 1 wherein said polymer is apolyparabanic acid.
 12. A method according to claim 1 wherein saidpolymer is a polysulfone.